William A. Bennett

Larval Striped Bass Condition in a Drought‐Stricken Estuary: Evaluating Pelagic Food‐Web Limitation

Estuarine food webs are frequently altered by human interventions, including freshwater diversions, toxic compounds, and introduced species. From 1988 through 1991 we examined the external morphological and internal histopathologic condition of larval striped bass (Morone saxatilis) to evaluate the potential importance of starvation to fish recruitment in the San Francisco Bay estuary. During a recent drought (1987-1992), fish populations declined markedly, concurrent with dramatic reductions in phytoplankton and zooplankton food for larval fishes. Such patterns suggest pelagic food is limited during times of low freshwater input; therefore, larval starvation may limit recruitment. However, toxic compounds in agricultural runoff are also less diluted in low-outflow years, enhancing their potential impact. Histopathology enabled us to identify their possible effects. In the laboratory, indices of larval morphology and eye and liver tissue condition reflected star- vation after 2 d of food deprivation. From 1988 through 1991 >90% of 980 field-caught specimens were classified morphologically as feeding larvae. Histopathological evaluation indicated that all field-caught specimens (N = 500) had food in their guts and lacked tissue alterations consistent with starvation. However, liver alterations consistent with toxic ex- posure were seen in 26-30% of the field-caught larvae from 1988 through 1990, dropping to 15% in 1991. While our findings implicate toxic exposure as a factor in the relationship between low freshwater input and poor year-class success of striped bass, reductions of toxic runoff and improvement in larval liver condition in 1991 did not improve larval survival. This suggests the potentially greater importance of interactions with food limitation and predation as well as the futility of pursuing single-factor explanations for recruitment failure. The potential obfuscation of food limitation by toxic exposure also indicates the need for interdisciplinary approaches to distinguishing anthropogenic intervention from estuarine food-web processes.

Otolith Microchemistry Provides Information Complementary to Microsatellite DNA for a Migratory Fish

Abstract We investigated the ability of otolith microchemistry to discriminate natal habitats of the splittail Pogonichthys macrolepidotus, a migratory cyprinid endemic to the San Francisco Estuary, California. Splittails are broadly distributed in the brackish and freshwater portions of the lower estuary and make long-distance upstream migrations during winter to rivers and floodplains for spawning. We found that the ratios of Sr: Ca and 87Sr: 86Sr in the otoliths (ascertained by laser ablation inductively coupled mass spectrometry) of age-0 fish collected from natal habitats significantly varied among four primary spawning rivers. Based on these two constituents, quadratic discriminant function analysis correctly classified 71% of the fish to their natal rivers. Recent work with microsatellite DNA indicates that splittails from these same rivers represent two genetically distinct populations. Thus, integrating data obtained from otolith microchemistry and microsatellite DNA can provide complementary inf...

Persistence of tidally-oriented vertical migration by zooplankton in a temperate estuary

Tidal vertical migration by zooplankton is a common phenomenon in estuaries, usually associated with landward movement of meroplankton or position maintenance of holoplankton. Little is known about the persistence of this behavior, its spatial variability, or its response to changing environmental conditions. We extended a previous study of tidal movements of zooplankton in the low-salinity zone (LSZ) of the San Francisco estuary in 1994 to include data from two additional years with very different hydrology. Freshwater flow during sampling in 1995 was about 7-fold greater than in 1994; the LSZ was about 28 km further seaward, and gravitational circulation in the LSZ was strong. In 1996 freshwater flow and LSZ position were intermediate but, because the LSZ was in shallower water in 1996 than in 1995, gravitational circulation was uncommon. Behavior of copepods in both years was similar to that reported in 1994 with some tidal migration observed during most cruises. An exception was the introduced carnivorous copepodTortanus dextrilobatus, which did not migrate and maintained a position deep in the water column (1995 only). In 1996, mysids mainly stayed near the bottom with evidence for vertical migration from only 1 of 6 data sets, whereas amphipods migrated slightly on a diel schedule; these behaviors contrasted with the tidal migration observed in 1994. The bay shrimpCrangon franciscorum did not appear to migrate, but was more abundant in the water column during both ebb and flood, suggesting passive vertical dispersal. Zooplankton did not appear to maintain position by interactions with lateral circulation cells. The results for copepods suggest rigidity in behavior with little or no relaxation of the vertical movement in 1995 when strong gravitational circulation would have made upstream movement relatively easy. Mysids and amphipods altered their behavior depending on local conditions related to freshwater flow.

Retrospective determination of natal habitats for an estuarine fish with otolith strontium isotope ratios

We investigated the ability of strontium isotope ratios ( 87 Sr/ 86 Sr) in otolith cores to record the natal habitats of juvenile delta smelt Hypomesus transpacificus from the San Francisco Estuary, USA. Young delta smelt (<60 days old) were collected during the California Department of Fish and Game 20-mm Survey in May and June of 1999 at several potential natal areas: Napa River, Suisun Marsh, West Delta, North Delta, Central Delta, South Delta and East Delta. The core region of sagittal otoliths was assayed with laser ablation-multicollector inductively coupled plasma mass spectroscopy. The laser ablation technique provided precise estimates of 87 Sr : 86 Sr ratios with relative standard deviation of 0.003% (one sigma). Isotope ratios ranged from 0.7065 to 0.708 and were different among natal habitats. However, natal habitats within the delta region were not discernable among each other, and reflect the mixing of the two major rivers, Sacramento River and San Joaquin River within the delta. We will therefore be able to determine natal habitats for delta smelt by assaying the core region of the otoliths. The application of strontium isotope ratios ( 87 Sr/ 86 Sr) in fish otoliths will greatly improve conservation efforts for this protected species.

Implications for Future Survival of Delta Smelt from Four Climate Change Scenarios for the Sacramento–San Joaquin Delta, California

Changes in the position of the low salinity zone, a habitat suitability index, turbidity, and water temperature modeled from four 100-year scenarios of climate change were evaluated for possible effects on delta smelt Hypomesus transpacificus, which is endemic to the Sacramento–San Joaquin Delta. The persistence of delta smelt in much of its current habitat into the next century appears uncertain. By mid-century, the position of the low salinity zone in the fall and the habitat suitability index converged on values only observed during the worst droughts of the baseline period (1969–2000). Projected higher water temperatures would render waters historically inhabited by delta smelt near the confluence of the Sacramento and San Joaquin rivers largely uninhabitable. However, the scenarios of climate change are based on assumptions that require caution in the interpretation of the results. Projections like these provide managers with a useful tool for anticipating long-term challenges to managing fish populations and possibly adapting water management to ameliorate those challenges.

ANALYSIS OF SIZE TRAJECTORY DATA USING AN ENERGETIC-BASED GROWTH MODEL

Individual growth rate of animals is increasingly used as an indicator of ecological stressors. Environmental contaminants often affect physiological processes with- in individuals, which in turn affect the animals growth rate. Consequently, there is an increasing need to estimate parameters in physiologically based individual growth models. Here, we present a method for estimating parameters in an energetic-based individual growth model (a dynamic energy budget model). This model is a system of stochastic differential equations in which one of the state variables (the energy reserve) is unobservable. There is no analytical solution to the probability density of size at given age, so we use a numerical nonlinear state-space method to calculate the likelihood. An algorithm to calculate the likelihood is outlined in this paper. This method is general enough to apply to other sto- chastic differential equation models. We assessed the estimability of parameters in the individual growth model, and analyzed size trajectory data from delta smelt ( Hypomesus transpacificus ). We expect this method to become an important tool in ecological studies as computers become faster, as the models that we deal with become more complex, and as the data that we collect become more detailed.

Classification of Larval and Adult Delta Smelt to Nursery Areas by Use of Trace Elemental Fingerprinting

Abstract Different environmental conditions among habitats may generate unique elemental patterns within fish otoliths that can be used to trace the life history as well as the potential natal origin of migratory species. We investigated the use of trace elements in otoliths as natural tags for determining the natal origins of larval and adult delta smelt Hypomesus transpacificus within a single estuary. Larval fish were collected at potential natal sites within the San Francisco Bay Estuary—the North, Central, South, and West Delta areas, Suisun Bay, and Napa River—during May–June 1999; adults were collected in November 1999 throughout Suisun Bay. Using laser ablation inductively coupled plasma mass spectrometry, we assayed trace elements from core (natal) regions of the otolith (Sr:Ca, Mg:Ca, and Ba:Ca ratios). Linear discriminant function analysis (LDFA) was 90.9–100% successful at classifying larval fish to their natal habitats (Napa River, Sacramento River, and Delta). Adults of unknown natal origin ...

Individual-Based Modeling of Delta Smelt Population Dynamics in the Upper San Francisco Estuary: II. Alternative Baselines and Good versus Bad Years

Abstract We used a previously described individual-based population model to further explore the population dynamics of Delta Smelt Hypomesus transpacificus in the upper San Francisco Estuary. We formulated four alternative baseline configurations of the model and used a factorial design to systematically isolate the effects of factors that determined a good versus bad year. The alternative baseline conditions were obtained by substituting different assumptions about growth, maturity, and mortality into the original baseline configuration. In the simulation experiment, we varied five factors by setting each value to its 1998 (best year) or 2001 (worst year) value: salinity, temperature, zooplankton densities, hydrodynamics, and eggs per age-1 individual at spawning. Although some of the alternative baselines resulted in lower January abundances, estimated finite population growth rates were very similar for all versions. The simulation experiment showed that juvenile growth in the winter prior to spawning...

Riders on the storm: selective tidal movements facilitate the spawning migration of threatened delta smelt in the San Francisco Estuary

Migration strategies in estuarine fishes typically include behavioral adaptations for reducing energetic costs and mortality during travel to optimize reproductive success. The influence of tidal currents and water turbidity on individual movement behavior were investigated during the spawning migration of the threatened delta smelt, Hypomesus transpacificus, in the northern San Francisco Estuary, California, USA. Water current velocities and turbidity levels were measured concurrently with delta smelt occurrence at sites in the lower Sacramento River and San Joaquin River as turbidity increased due to first-flush winter rainstorms in January and December 2010. The presence/absence of fish at the shoal-channel interface and near the shoreline was quantified hourly over complete tidal cycles. Delta smelt were caught consistently at the shoal-channel interface during flood tides and near the shoreline during ebb tides in the turbid Sacramento River, but were rare in the clearer San Joaquin River. The apparent selective tidal movements by delta smelt would facilitate either maintaining position or moving upriver on flood tides, and minimizing advection down-estuary on ebb tides. These movements also may reflect responses to lateral gradients in water turbidity created by temporal lags in tidal velocities between the near-shore and mid-channel habitats. This migration strategy can minimize the energy spent swimming against strong river and tidal currents, as well as predation risks by remaining in turbid water. Selection pressure on individuals to remain in turbid water may underlie population-level observations suggesting that turbidity is a key habitat feature and cue initiating the delta smelt spawning migration.

Individual-Based Modeling of Delta Smelt Population Dynamics in the Upper San Francisco Estuary: I. Model Description and Baseline Results

Abstract Many factors have been implicated in the decline of Delta Smelt Hypomesus transpacificus in the upper San Francisco Estuary, and the importance of each factor is difficult to determine using field data alone. We describe a spatially explicit, individual-based population model of Delta Smelt configured for the upper estuary. The model followed the reproduction, growth, mortality, and movement of individuals over their entire life cycle on the same spatial grid of cells as the Delta Simulation Model (DSM2) hydrodynamics model. Daily values of water temperature, salinity, and densities of six zooplankton prey types were represented on the spatial grid. Reproduction was evaluated daily, and new individuals were introduced into the model as yolk sac larvae. Growth of feeding individuals was based on bioenergetics and zooplankton densities. Mortality sources included natural mortality, starvation, and entrainment in water diversion facilities. Movement of larvae was determined using a particle tracking...